When the stimulable phosphor is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The stimulable phosphor then emits stimulated emission according to the level of the stored energy when the phosphor is exposed to electromagnetic wave such as visible light or infrared rays (i.e., stimulating light).
A radiation image recording and reproducing method utilizing the stimulable phosphor has been widely employed in practice. The method employs a radiation image storage panel comprising the stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the storage panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with a stimulating light to emit stimulated light; and photo-electrically detecting the emitted light to obtain electric signals giving a visible radiation image. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as stimulable phosphor sheet) has a basic structure comprising a support (or substrate) and a stimulable phosphor layer provided thereon.
The phosphor layer is generally formed by coating a dispersion of phosphor particles in a binder solution on the support and drying the coated dispersion, and therefore comprises a binder and phosphor particles dispersed therein.
It is desired that radiation image storage panels used in these methods have sensitivity as high as possible.
It is known that a radiation image storage panel having on a support a stimulable phosphor film prepared by gas phase deposition such as vacuum vapor deposition or sputtering gives a reproduced radiation image with high sensitivity as well as high sharpness.
There is already known a method in which a stimulable phosphor layer of a radiation image storage panel is formed by electron beam evaporation (which is a kind of vapor deposition method). In the method, an electron beam generated by an electron gun is applied onto a stimulable phosphor or its starting materials (i.e., evaporation source) to heat and vaporize the source, to deposit the vapor to form a phosphor layer on the surface of the support. Thus formed phosphor layer consists essentially of prismatic crystals of the stimulable phosphor. In the phosphor layer, there are cracks among the prismatic crystals of the stimulable phosphor. For this reason, the stimulating rays are efficiently applied to the phosphor and the stimulated emission are also efficiently taken out. Hence, a radiation image of high sharpness can be obtained with high sensitivity.
Japanese Patent No. 3,041,717 describes a process for preparing a radiation image storage panel which comprises the steps of depositing on a support a layer of stimulable phosphor or stimulable phosphor matrix component having a thickness of 200 to 600 μm, and heating the deposited layer at T° C. (0.40Tm<T<0.75Tm, Tm is a melting point of the stimulable phosphor matrix component) for 2 to 10 hours in an atmosphere containing the activator component.
Japanese Patent Publication No. 7-18957 discloses a process for preparing a radiation image storage panel comprising the steps of depositing a layer of stimulable phosphor comprising an activated stimulable phosphor and subsequently heating the deposited layer in the presence of a vapor of the activator component vaporized from the activator component source placed within the deposition vessel but far from the deposited layer.
In both of the above-mentioned processes, the explicitly described temperature for the heating step is a high temperature of 300° C. to 600° C. Further, the explicitly described stimulable phosphor is RbBr:Tl. The activator component Tl of this phosphor has a vapor pressure higher than the vapor pressure of RbEr. Therefore, the activator component is apt to leave from the deposited phosphor layer, and hence the heating step should be carried out in an atmosphere containing a gaseous activator component.
Japanese Patent Publication No. 6-54360 discloses a process for forming a stimulable phosphor layer containing an activator component which comprises the steps of depositing a stimulable phosphor matrix component layer and subsequently depositing on the matrix component layer the activator component so as to diffuse into the matrix layer. The activator component layer can be placed on the matrix layer otherwise, the matrix layer can he heated in the presence of a gaseous activator component.